Debian Configuration :: Disable Global User List ?

When I did external vulnerability scan of my servers, I discovered the following issue that I must correct:

THREAT:

This is the global system user list, which was retrieved during the scan by exploiting one or more vulnerabilities.

IMPACT:

These common account(s) can be used by a malicious user to break-in the system via password bruteforcing.

SOLUTION:

To prevent your host from being attacked, do one or more of the following:

RESULT:

So.. How can I prevent this scanner to reveal user names, in my case root ? Please help, It is mandatory for me to correct these issue.